1. The Field of the Invention
The present invention relates generally to gathering and analyzing measurements from an air conditioning system. In particular, the invention relates to systems and methods for diagnosing a status of an air conditioner using sensors, transmitters, receivers, and a computing device.
2. The Relevant Technology
The concept of air conditioning dates back at least to the first millennium B.C., when the ancient Romans cooled houses by circulating water through their walls. Modern air conditioning technology, which controls not only air temperature but also air humidity, emerged during the first decade of the 1900's. Throughout the first half of the 20th century, this technology was used primarily to improve productivity and control processes in industrial settings, such as printing plants and textile factories.
Residential use of air conditioners increased significantly in the 1950's. As the cost of air conditioning technology decreased, residential air conditioners spurred growth and development in cities with hot, dry climates, such as Phoenix, Ariz. and Las Vegas, Nev. Air conditioning also played an important role in improving the living conditions in the southeastern United States, where high temperatures and high humidity are common during the summer months.
A modern air conditioning system uses ducts and a fan to circulate air throughout a building while leveraging the evaporation cycle of a refrigerant to lower the temperature and humidity of the air as it passes through the ducts. The ducts include return ducts and supply ducts. The return ducts take in warm air from the living space and circulate the warm air across an evaporating coil. As the warm air passes over the evaporating coil, the refrigerant circulating through the evaporator absorbs heat and moisture from the air, lowering the air's temperature and humidity. The supply ducts then circulate the cool air back to the living space.
The condensing unit controls the evaporation cycle of the refrigerant. Located outside the building, the condensing unit includes a compressor, condensing coils, and an expansion device. Together, the condensing unit and the evaporating coil constitute a closed system through which the refrigerant circulates. The refrigerant leaves the evaporator and enters the compressor as a cool gas. The compressor compresses the gas, transforming it into a hot gas. Heat dissipates as the hot gas leaves the compressor and passes through the condensing coils, causing the hot gas to condense into a cool liquid. As the cool liquid passes through the expansion device, the expansion device reduces the pressure on the refrigerant, transforming it into a cold gas or liquid. The cold gas or liquid then flows through the evaporator, where it can absorb heat and moisture from the air circulating through the ducts.
Air conditioning systems may malfunction or perform poorly for a variety of reasons. Dirty filters can reduce the air flow through the ducts, forcing the air conditioner to run longer to condition a given volume of air. Dirty coils can reduce the rate of transfer of heat between the refrigerant and the air. Leaky or poorly insulated ducts can transfer heat from an attic into the circulating air or vent cool air into a crawlspace. Using too much refrigerant (“overcharging”), which is typically the result of improper maintenance, reduces the efficiency of the air conditioning system and may damage the condenser. Likewise, using too little refrigerant (“undercharging”), which is typically the result of a leak or improper maintenance, reduces the efficiency of the air conditioning system.
Overcharging or undercharging of the refrigerant is a serious problem requiring significant expertise to diagnose and correct. Simple viewing windows that permit a person to observe the level of refrigerant in some component of the air conditioning system are unreliable because the amount of refrigerant in a given component of the system varies when the system is operating, and the refrigerant tends to collect in the coolest component of the system when the system is not operating. Diagnostic methods that rely on draining or venting the refrigerant are discouraged because refrigerants used in air conditioners are often environmentally unsafe. Therefore, diagnostic methods that rely on electronic sensors to indirectly measure refrigerant levels are safer and more reliable than manual methods.
However, typical sensor-based systems that indirectly measure refrigerant levels leave much room for improvement. In particular, typical systems tend to gather inaccurate data, and, as a consequence, these systems often mischaracterize the status of the refrigerant. Typical systems gather inaccurate data for several reasons.
Typical data-gathering systems measure, among other quantities, the temperature of the air in the supply and in the return, as well as the humidity of the air in the return. Parameters of the air in the supply and in the return can be sampled at many locations, because the supply and the return are both large volumes of air. However, parameters of the supply are more accurately measured as the distance between the sampling location and the evaporator decreases. Typical data-gathering systems often acquire inaccurate measurements because they are unable to sample parameters of the supply at a location sufficiently near the evaporator.
A typical data-gathering system uses a single transmitter deployed in the interior of the building to measure the air temperatures at the return and the supply. This single transmitter may be equipped with two temperature sensors attached via cables, but it is often difficult or impossible to position those two sensors such that they simultaneously and accurately measure the temperatures of the air in the return and the air in the supply, because the distance between two positions where accurate measurements can be obtained is often quite large.
Furthermore, a typical data-gathering system uses temperature sensors that are too large to fit between the slats of a vent. Consequently, the typical system samples the air temperatures in front of the return and supply vents, rather than behind the vents. For these reasons, typical data-gathering systems are often unable to accurately measure the air temperature in the return and in the supply nearest the evaporator.
Improper calibration of sensors is another common source of inaccurate data in typical data-gathering systems. In a typical data-gathering system, the manufacturer calibrates the sensors and distributes calibration files that are stored external to and remote from the sensor/transmitter units. With this calibration scheme, sensors are often calibrated incorrectly. Incorrect calibration of sensors can yield wildly inaccurate sensor readings. Thus, miscalibrated sensors are another source of inaccurate data in typical data-gathering systems.
To compensate for inaccurate measurements, typical data-gathering systems average multiple readings of the same sensor to obtain an estimate of the sensed value for a given sampling period. Averaging reduces but does not eliminate the impact of inaccurate, outlying measurements, such as those measurements that might be obtained by miscalibrated sensors.
The accuracy of the refrigerant status determined by the sensor-based system depends greatly on the accuracy of the data gathered by the sensors. When the gathered data is inaccurate, the status determined by the system tends to be inaccurate. In the worst case, when the gathered data includes many inaccurate, outlying measurements, the status determined by the system tends to oscillate between “overcharged” and “undercharged”. This instability makes it difficult for a service technician to properly charge the refrigerant or to determine when the refrigerant is properly charged.
Typical data-gathering systems also suffer from power supply problems. These systems tend to use transmitters with non-standard batteries that cannot be purchased at local stores. If rechargeable, these batteries typically must be removed from the transmitters for recharging. Also, the transmitters tend to waste considerable energy between successive data transmissions. Taken together, these factors create conditions under which the transmitters' energy sources are easily depleted and not easily replenished.
Typical data-gathering systems use an RS-232 protocol for communication between the receiver and the computer. However, RS-232 ports are uncommon on modern computers.
Typical data-gathering systems also use long external antennae that attach to the exteriors of the transmitters. However, these exterior antennae are easily lost or broken, rendering the transmitters inoperable.